1. Field of the Invention
The present invention relates to a method for manufacturing a flash memory device and a flash memory device manufactured by the same, and more specifically, to a method for manufacturing a flash memory device and a flash memory device manufactured by the same, which can improve the quality of a tunnel oxide film.
2. Discussion of Related Art
Recently, there is an increasing demand for a flash memory device that can be electrically programmed and erased and has a refresh function of rewriting data in a given period. Research for higher-integration technology of a memory device has been actively made in order to develop a large-capacity memory device capable of storing lots of data therein. In this case, programming refers to an operation of writing data into the memory cell and erasure refers to an operation of erasing data written into the memory cell.
For higher-integration of a memory device, an NAND-type flash memory device in which a plurality of memory cells are serially connected (i.e., structure in which a drain or a source among neighboring cells are shard) to form a single string, The NAND-type flash memory device is a memory device from which information is sequentially read unlike a NOR-type flash memory device. The programming and erasure operations of the NAND-type flash memory device are performed by controlling the threshold voltage Vt of the memory cell by injecting or discharging electrons into or from a floating gate through F-N tunneling scheme.
In the NAND-type flash memory device, securing reliability of the memory cell is an integral problem. In particular, the data retention properties of the memory cell come to the front as an important problem. As described above, however, in the NAND-type flash memory device, the programming and erasure operations are performed through F-N tunneling scheme. In such repetitive F-N tunneling process, electrons are trapped in the tunnel oxide film of the memory cell, which causes the threshold voltage Vt of the memory cell to shift. Thus, there occurs a case where data originally stored in the memory cell may be erroneously recognized when reading the data. That is, there occurs a problem that reliability of the memory cell is degraded.
Shift in the threshold voltage of the memory cell is caused by electrons trapped in the tunnel oxide film by means of repetitive F-N tunneling process due to cycling. In the above, the cycling refers to a process of repeatedly performing the programming operation and the erasure operation. In order to prevent the shift in the threshold voltage of the memory cell, there was proposed a method for reducing an erasure voltage sufficiently below a verify voltage by controlling a bias condition (i.e., bias voltage) upon programming and erasure. In this method, however, the threshold voltage is increased as much as the bias voltage. This still poses a problem that the threshold voltage shifts. As another method for preventing shift in the threshold voltage of the memory cell, there is a method for reducing a thickness of the tunnel oxide film to reduce the amount of electrons trapped at the time of F-N tunneling. This method for reducing the thickness of the tunnel oxide film, however, has a limit due to a fundamental data retention quality problem or a read disturbance problem.